Investigation of AlGaN Channel HEMTs on β-Ga₂O₃ Substrate for High-Power Electronics

Abstract

The wider bandgap AlGaN (Eg > 3.4 eV) channel-based high electron mobility transistors (HEMTs) are more effective for high voltage operation. High critical electric field and high saturation velocity are the major advantages of AlGaN channel HEMTs, which push the power electronics to a greater operating regime. In this article, we present the DC characteristics of 0.8 µm gate length (L_G) and 1 µm gate-drain distance (L_GD) AlGaN channel-based high electron mobility transistors (HEMTs) on ultra-wide bandgap β-Ga₂O₃ Substrate. The β-Ga₂O₃ substrate is cost-effective, available in large wafer size and has low lattice mismatch (0 to 2.4%) with AlGaN alloys compared to conventional SiC and Si substrates. A physics-based numerical simulation was performed to investigate the DC characteristics of the HEMTs. The proposed HEMT exhibits sheet charge density (n_s) of 1.05 × 10¹³ cm⁻², a peak on-state drain current (I_DS) of 1.35 A/mm, DC transconductance (g_m) of 277 mS/mm. The ultra-wide bandgap AlGaN channel HEMT on β-Ga₂O₃ substrate with conventional rectangular gate structure showed 244 V off-state breakdown voltage (V_BR) and field plate gate device showed 350 V. The AlGaN channel HEMTs on β-Ga₂O₃ substrate showed an excellent performance in I_ON/I_OFF and V_BR. The high performance of the proposed HEMTs on β-Ga₂O₃ substrate is suitable for future portable power converters, automotive, and avionics applications.

1. Introduction

Group III-nitride-based heterostructure devices showed excellent performance in high-power switching electronics [1,2,3]. Owing to the unique material properties of GaN such as high breakdown field (3.3 MV/cm), high mobility (>1200 cm²/v.s), and wide bandgap (3.4 eV), the GaN-based high electron mobility transistors (HEMTs) are exhibited high 2DEG density (two-dimensional electron gas), low on-resistance (R_on), low conduction loss, and high breakdown voltage (V_BR). Which enable the devices for high-temperature and high-power switching applications [4,5]. However, it is difficult to obtain high breakdown voltage for GaN-channel based HEMTs for smaller dimensions for portable high power switching applications like power converters, automotive electronics, and avionics. Therefore, further enhancing the power handling capability of the HEMTs, the ultra-wide bandgap (>3.4 eV) Al_xGa_1−xN channel-based devices are widely used [6,7,8,9,10,11]. There is a wide bandgap and superior thermal stability in AlGaN material adopted as a channel for improving the high-power handling and high-temperature operations of HEMTs. Gate length 2 µm (L_G) AlN/Al_0.5Ga_0.5N HEMT on sapphire substrate demonstrated remarkable breakdown voltage improvements [6]. L_G = 0.8 µm and L_GD = 1 µm Al_0.31Ga_0.69N/Al_0.1Ga_0.9N double-channel HEMTs recorded a V_BR of 143.5 V [8]. However, the device has shown a large negative threshold voltage (V_TH = −11.2 V), which will increase the power loss of the device at off-state. An Al-rich AlN/Al_0.85Ga_0.15N HEMTs on sapphire substrate showed 810 V of breakdown voltage (V_BR) for a gate-drain distance of (L_GD) of 10 µm [9]. Another Al-rich AlN/Al_0.65Ga_0.35N HEMTs on the sapphire substrate demonstrated 770 V off-state blocking voltage for 9 µm L_GD [10]. L_G = 3 µm Al_0.3Ga_0.7N/Al_0.1Ga_0.9N heterostructure on sapphire substrate exhibited excellent sheet carrier density and electron mobility [11], and hybrid ohmic/Schottky drain device configuration showed excellent off-state breakdown voltage. Despite high off-state blocking voltage of Al-rich AlGaN channel HEMTs [6,9,10], the current driving capability is very low due to alloy disorder scattering effects. On other hand, low Al composition AlGaN channel-based HEMTs [8,11] exhibited excellent sheet charge density and carrier mobility.

Apart from channel engineering, substrate engineering is one of the key issues in group III-nitride-based HEMTs. Sapphire (13% lattice mismatch with GaN), SiC (3.8% lattice mismatch with GaN), and Si (16% lattice mismatch with GaN) are widely used as substrate materials for GaN-based HEMTs [1,2,3,4,5,6,7,8,9,10,11]. Due to lattice mismatch between buffer and substrate materials, an additional AlN layer is required in between for lattice match, which involves additional costs and increases the device fabrication complexity.

Recently, β-Ga₂O₃ is an emerging material for power electronics applications. The wide bandgap (4.7 eV), high critical electric field (6–8 MV/cm), large-scale, high-quality bulk substrate, low defect density, and nearly lattice match (0 to 2.4% lattice mismatch for AlGaN alloys) with III-Nitride alloys make β-Ga₂O₃ a promising material for future high-power applications [12,13,14,15,16,17,18]. A numerical study on normally-off AlN/β-Ga₂O₃ based HEMT with GaN back barrier showed a suppressed leakage current and excellent DC characteristics [19]. In spite of high-power performance, the β-Ga₂O₃ channel-based HEMTs experienced low electron mobility and low sheet charge concentration [18] as a result of low current driving capability. In this work, we used β-Ga₂O₃ as a substrate for the following reasons. 1. Low lattice mismatch with AlGaN alloys and therefore an additional thick AlN nucleation layer is not required 2. Relatively low cost compared to GaN and SiC, and 3. Availability of large wafer size. The novelty of the work is gate field plate AlGaN channel HEMTs on β-Ga₂O₃ substrate.

The first Al_0.1Ga_0.9N channel-based HEMTs on the β-Ga₂O₃ substrate is proposed in this work for simultaneous improvement in both power handling and current driving capability of HEMTs. The proposed gate field plate Al_0.31Ga_0.69N/Al_0.1Ga_0.9N HEMTs on β-Ga₂O₃ substrate is investigated using ATLAS TCAD [20] and the device DC characteristics are presented. The device simulation models are validated with the experimental results. The field plate gate low Al composition (Al = 10%) AlGaN channel HEMT on β-Ga₂O₃ substrate showed remarkable improvement in breakdown voltage (V_BR), on-state drain current density, and low on-resistance.

2. Device Structure and Simulation Model

Figure 1a shows the schematic cross-section of the conventional gate Al_0.1Ga_0.9N channel HEMT (Device A) on β-Ga₂O₃ substrate and field plate gate HEMT (Device B) is displayed in Figure 1b. Both device’s epi-stack consist of 100 nm Al_0.1Ga_0.9N channel, 23 nm Al_0.31Ga_0.69N barrier, and 2.2 µm Al_0.31Ga_0.69N buffer, 100 nm SiN passivation (Ɛ_r~7). Low Al-composition Al_0.1Ga_0.9N channel is used in this to improve the sheet charge density (2DEG) and enhance electron mobility (µ) [8,11]. The Al_0.1Ga_0.9N layer is directly grown on the β-Ga₂O₃ substrate. A low lattice mismatch (<2.4%) between AlGaN buffer and β-Ga₂O₃ substrate alleviates the interface defects and enhanced the device performance. Moreover, the β-Ga₂O₃ wafer is available in a larger size and low cost compared with SiC and GaN [12,13,14,15,16,17,18]. Device gate length is defined as 0.8 µm (L_G) and asymmetrical gate to source (L_GS = 0.8 µm) and gate to drain distance (L_GD = 1 µm) is considered for this work in order to reduce the source resistance. To analyze the impact of filed plate structure on the breakdown performance of the AlGaN channel HEMT, a 0.75 µm field plate (L_FP) was used in Device B. Ohmic contact was realized for source and drain contact by defining the work function of the electrode as 3.4 eV and Schottky gate contact enabled by defining electrode work function as 5.2 eV. Field plate gate structure used for the proposed HEMT for improving the off-state blocking voltage of the device by re-shaping the electric field across the gate to the drain access region. The SiN passivation avoids the surface-related trapping effects and avoids the current collapse phenomena.

The TCAD simulation energy band diagram is plotted in Figure 2 at zero gate bias and drain bias. It shows the conduction band discontinuity at the interface of Al_0.31Ga_0.69N/Al_0.1Ga_0.9N and forms a 2DEG due to spontaneous and piezoelectric polarization effects. The sheet charge density (n_s) of 1.05 × 10¹³ cm⁻² extracted from the device simulation. The corresponding polarization charge details are displayed in Figure 3. The material parameters for the simulation are taken from [16,17,18,19,20,21,22] and are presented in

Table 1. Material parameters used for the TCAD simulation.

Material Parameters	GaN	AlN	β-Ga2O3
Bandgap, Eg (eV)	3.4	6.2	4.85
Dielectric constant, Ɛ	9	8.3	10
Electron mobility, µ (cm2/V.s)	1250	300	300
Saturation velocity, v (cm/s)	2.5	1.6	1.8
Thermal conductivity, λ (W/cm.K)	2.3	2.85	0.3

.

The basic device physics models that operate on any semiconductor devices have been derived from Schrodinger’s equation, Shockley–Read–Hall (SRH) recombination model, Poisson’s equations, the continuity equations, and the transport equations. The quantum electron density relies on the one-dimensional (1D) Schrodinger’s equation, which relates the eigen state energies $E_{iv}\left(x\right)$ and wave function ${\phi }_{iv}\left(x,y\right)$.

$$-\frac{h^2}{2}\frac{\partial }{\partial y}\left(\frac{1}{m_y^v\left(x,y\right)}\frac{\partial {\phi }_{iv}}{\partial y}\right)+E_C\left(x,y\right){\phi }_{iv}=E_{iv}{\phi }_{iv}$$

(1)

where $m_y^v\left(x,y\right)$ indicates spatial dependent effective mass and $E_C\left(x,y\right)$ represents the conduction band edge. Poisson’s equations relate the electric field (E), electrostatic potential ($\mathsf{\psi }$), and space charge concentration ($\rho$). Which is written as follows [20];

$${\nabla }^2\mathsf{\psi }=-\nabla E=\frac{\rho }{\epsilon }$$

(2)

The continuity and transport equations describe the time-dependent electron (n) and hole (p) densities due to carrier transport, carrier generation ($G_n$ and $G_p$), and carrier recombination ($R_n$ and $R_p$) process. The continuity equations are defined by the following equations [20];

$$\frac{\partial n}{\partial t}=\frac{1}{q}\nabla J_n+G_n-R_n$$

(3)

$$\frac{\partial p}{\partial t}=-\frac{1}{q}\nabla J_p+G_p-R_p$$

(4)

The drift-diffusion carrier transport model is based on Boltzmann transport theory, which relates the current densities ($J_n$ and $J_p$) with quasi-Fermi levels (${\varnothing }_n$ and ${\varnothing }_p$) and the quasi-Fermi levels are related with carrier concentrations and potentials through Boltzmann approximations [20].

$$J_n=-q{\mu }_{n}n\nabla {\varnothing }_n$$

(5)

$$J_p=-q{\mu }_{p}p\nabla {\varnothing }_p$$

(6)

The polarization property of group III-nitride is the major source for 2DEG, the spontaneous (P_SP) and piezoelectric polarization (P_PZ) models are used for the device simulation. The carrier mobility impacts the device on-state current density (I_DS) and transconductance (g_m). Therefore, the temperature, composition, and doping dependent Farahmand Modified Caughey Thomas low field mobility model (FMCT.N) and nitride specific high field mobility (GANSAT.N) models [20] are used in this work. In order to capture the trap-assisted recombination due to crystal defects and dopant, Shockley–Read–Hall (SRH) recombination model is used for the device simulation. Which is described by the following equations [20];

$$R_{net}^{SRH}=\frac{n-n_{ie}^2}{{\tau }_p\left[p+n_{ie}\exp \left(\frac{-E_{trap}}{KT}\right)\right]+{\tau }_n{\left[p+n_{ie}\exp \left(\frac{-E_{trap}}{KT}\right)\right]}^{\prime }}$$

(7)

where ${\tau }_p$ and ${\tau }_n$ are electron and hole lifetime, respectively, and $E_{trap}$ defines the intrinsic Fermi energy level to trap energy level. The impact ionization model described by Selberherr used for device breakdown simulation, which relates the carrier generation ($G$) rate with electron (${\alpha }_n$) and hole ionization coefficients (${\alpha }_p$), and electron ($J_n$) and hole current densities ($J_p$) [20];

$$G={\alpha }_p\left|J_p\right|+{\alpha }_n\left|J_n\right|$$

(8)

3. Results and Discussions

The current driving ($J=qv{n}_s$) capability of the HEMTs relies on the electron velocity ($v$), sheet charge, and 2DEG density (n_s). The proposed HEMT showed 1.12 × 10⁷ cm/s of electron velocity and sheet charge density (n_s) of 1.05 × 10¹³ cm⁻². The DC characteristics of proposed AlGaN channel HEMTs on β-Ga₂O₃ are presented in this section. The device output characteristics is plotted in Figure 4 for V_GS = −4 V to 8 V and V_DS from 0 V to 20 V. The HEMT showed a peak on-state drain current (I_DS) of 1.35 A/mm at V_GS = 2 V and the extracted ON-resistance (R_ON = ∆V_DS/∆I_DS) from the V-I curve corresponding to V_GS = 0 V is 8.83 Ω.mm. The DC transfer characteristics of HEMT is displayed in Figure 5a at V_DS = 10 V and V_GS swept from −10 V to 8 V. A peak transconductance (g_m) of 277 mS/mm obtained at V_GS = −1.5 V and extracted a threshold voltage of −4.31 V. The device transfer characteristics log-scale plot displayed in Figure 5b and the HEMT showed an excellent I_ON/I_OFF ratio of ~10¹⁴. The obtained I_DS and g_m are the highest results among the Al_xGa_1−xN channel HEMTs [6,7,8,9,10,11]. The TCAD simulation is validated with the L_G = 0.8 µm gate length AlGaN channel HEMT experimental results for transfer and breakdown performance [8] and depicted in Figure 6, which shows the simulation results are well correlated with experimental results.

For power electronics applications, low ON-resistance (R_ON) and high breakdown voltage (V_BR) are the most critical parameters to minimize the resistive losses during high power operation of HEMTs, which determine the efficiency of the power devices. In III-nitride-based HEMTs, the 2DEG is induced by polarization, and the breakdown voltage depends on the gate-to-drain distance (L_GD). The ON-resistance of the HEMT expressed as following expressions [23];

$$R_{on}=R_{channel}+R_{drain}=\frac{L_G}{W_G}·\frac{1}{q\mu n_s}+\frac{L_{GD}}{W_G}·\frac{1}{q\mu n_s}$$

(9)

where $R_{channel}$ is the channel resistance and $R_{drain}$ is drain side access resistance. Primarily, the off-state breakdown voltage depends on $L_{GD}=\frac{V_{BR}}{E_{crit}}$ when source and drain contact resistances are neglected. In general, longer gate length and larger gate-drain spacing HEMTs typically exhibit high V_BR but the ON-resistance also increased with $L_G$ [23]. Shorter gate length and small $L_{GD}$ HEMTs showed low breakdown voltages. Hence, the optimization of HEMTs structure is the key component for achieving high breakdown field and low R_ON for future low loss power semiconductor devices with smaller device sizes. In this work, we used L_G = 800 nm gate length and L_GD = 1000 nm ultra-wide bandgap AlGaN channel along with gate field structure. In addition, the obtained results were compared with existing HEMTs with same L_G and L_GD [8].

The high critical electric field of AlGaN channel (~4×) HEMTs to GaN-channel HEMTs leads to high voltage operation of the device. Figure 7 shows the forward blocking characteristics of the conventional AlGaN channel and gate field plate AlGaN channel HEMTs. The off-state breakdown voltage (V_BR) is extracted from V_D-I_D when drain leakage current reaches 1 mA/mm. As shown in Figure 7, L_G = 0.8 µm conventional AlGaN channel HEMT on β-Ga₂O₃ substrate demonstrated 244 V of V_BR and L_G = 0.8 µm and L_FP = 0.75 µm gate field plate HEMT on β-Ga₂O₃ substrate showed a V_BR of 350 V. The field plate HEMT modulates the electric field in the device access regions (gate-drain space) and able to sustain high electric fields, which leads to enhanced breakdown voltage. Logarithmic electron concentration of the HEMTs is displayed in Figure 8 at the breakdown condition. The Depletion region of gate field plate HEMT in Figure 8b is larger than the conventional gate HEMT Figure 8a. This represents that the higher depletion width leads to high V_BR and the smaller depletion width would decrease the V_BR of the HEMT. Gate field plate AlGaN channel HEMT on β-Ga₂O₃ substrate exhibited excellent breakdown characteristics and low ON-resistance than existing AlGaN channel HEMT with identical device size [8]. This reveals the potential of AlGaN channel HEMTs on β-Ga₂O₃ substrate for future power switching applications with smaller device sizes. The comparison of proposed AlGaN channel HEMTs breakdown performance with previously reported AlGaN channel HEMTs [8,9,10,11,24,25,26,27,28,29] are presented in Figure 9. Our device showed excellent breakdown voltage than existing HEMTs for smaller gate length (L_G) and gate-drain distance (L_GD).

The tentative fabrication steps of the proposed HEMT are depicted in Figure 10. The epitaxial growth can be performed by the MOCVD process (Metal Organic Chemical Vapor Deposition) on the β-Ga₂O₃ substrate. Device mesa isolation will reduce the reducing leakage currents. This can be achieved through dry etching ICP-RIE (Inductively Coupled Plasma Reactive Ion Etching). The patterned deposition of source and drain contacts will be conducted through optical lithography and e-beam evaporation followed by high-temperature annealing. The minimum feature size used is 1 micron hence patterning is possible through optical lithography for lower feature sizes one has to depend on e-beam lithography. Then, patterning and deposition of gate metal followed by contact annealing at a lower temperature to ensure Schottky nature of the contact. Blanket deposition of silicon nitride through PECVD (Plasma Enhanced Chemical Vapor Deposition) and ALD (Atomic Layer Deposition) over the entire substrate. Selective etching of SiN for source, gate, and drain pad openings through ICP-RIE.

4. Conclusions

AlGaN channel HEMTs on β-Ga₂O₃ substrate are proposed and the device DC characteristics with rectangular and gate field plate structures are analyzed. The wide-bandgap AlGaN channel along with field plate gate HEMT showed improved breakdown voltage (V_BR) of 350 V for L_G = 0.8 µm and L_GD = 1 µm, whereas the conventional gate HEMT with identical device dimensions showed 244 V of V_BR. The improved breakdown voltage from the larger depletion region at breakdown condition in field plate HEMT. The low Al composition Al_0.1Ga_0.9N channel also demonstrated a peak ON-state current density of 1.35 A/mm, 8.83 Ω.mm of R_ON, and g_m of 277 mS/mm. TCAD simulation and DC characteristics of the β-Ga₂O₃ substrate-based AlGaN channel HEMTs indicates the suitability of proposed HEMTs for future portable less weight power converters, motor driver, consumer electronics, automotive, and avionics applications. In addition, the β-Ga₂O₃ substrate can be utilized for all group III-nitride-based transistors for better performance.